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Abstract Nitrite is a ubiquitous compound found across aquatic systems and an intermediate in both the oxidative and reductive metabolisms transforming fixed nitrogen in the environment. Yet, the abiotic cycling of nitrite is often overlooked in favor of biologically mediated reactions. Here we quantify the apparent acid dissociation constant (pK_a) between nitrous acid and its conjugate base nitrite in both freshwater and seawater systems across a range of environmentally relevant temperatures (5–35°C) using potentiometric‐based titration. In freshwater, we measured a pK_a,NBSof 3.14 at 25°C and a pK_a,Tof 2.87 for seawater at the same temperature. We quantify substantial effects of both salinity and temperature on the pK_a, with colder and fresher water manifesting higher values and thus a greater proportion of protonated nitrite at any given pH. Because nitrous acid is unstable and decomposes to nitric oxide, the implications for the nitrous acid dissociation constant on ecosystem function are broad. 

Perchlorate (ClO4–) is a pervasive, harmful, and inert anion on both Earth and Mars. Current technologies for ClO4– reduction entail either harsh conditions or multicomponent enzymatic processes. Herein, we report a heterogeneous (L)Mo–Pd/C catalyst directly prepared from Na2MoO4, a bidentate nitrogen ligand (L), and Pd/C to reduce aqueous ClO4– into Cl– with 1 atm of H2 at room temperature. A suite of instrument characterizations and probing reactions suggest that the MoVI precursor and L at the optimal 1:1 ratio are transformed in situ into oligomeric MoIV active sites at the carbon–water interface. For each Mo site, the initial turnover frequency (TOF0) for oxygen atom transfer from ClOx– substrates reached 165 h–1. The turnover number (TON) reached 3840 after a single batch reduction of 100 mM ClO4–. This study provides a water-compatible, efficient, and robust catalyst to degrade and utilize ClO4– for water purification and space exploration.